1. Field of the Invention
The present invention relates generally to gain adjuster circuits and, more particularly, to a gain adjuster circurt for adjusting the gain for a plurality of photomultiplier tubes in one system.
2. The Prior Art
There are a variety of instruments which use photomultiplier tubes (PMT's). Some of these instruments include gamma-cameras, ct scanners, spectrum analysers and the like. One specific radioactivity distribution system employing a plurality of PMT's is disclosed in my prior U.S. Pat. No. 4,267,452 assigned to a common assignee, which patent is incorporated herein by reference.
A photomultiplier tube (PMT) is a device which employs secondary emission from solids to produce current amplification. The PMT has one or more dynodes between its photocathode and its output electrode. The bombarding electrons initiating a cascade among the dynodes are due to photo-emission, i.e., the emission of electrons from a substance forming the photocathode by subjecting it to electromagnetic radiation such as light, X-rays and the like. The number of secondary electrons released depends upon the kinetic energy of the primary (the bombarding) electrons and, therefore, upon the voltage whereby the primary electrons are accelerated. Thus, PMT's are highly voltage sensitive. The gain of a PMT changes very rapidly with a relatively small change in the applied voltage across the PMT.
In a system employing a plurality of PMT's, the adjustment of the gain of these PMT's is effected by adjusting the individual voltages for each PMT. Originally, the adjustment of the voltage for each PMT was done by manually adjusting a potentiometer associated with each PMT. A later method uses a digital to analog converter to adjust the voltage for each PMT. The digital to analog converter receives its input from an external controller, such as a microprocessor.
Each of the above two methods is less than desirable and both are cumbersome and expensive, particularly so when the system uses a large number of PMT's. A need exists, therefore, for a simple method to adjust the voltage for each PMT in a system employing a plurality of PMT's.